Widespread employment is made by the construction and utilities industry, particularly the latter, of vehicles having mounted thereon extensible and articulated arms for the purpose of placing men and equipment above ground level for performing some work related to the particular industry. The utilization of the articulated extensible arms permits the employment of a vehicle with a low profile for travelling from job to job and yet provides the necessary means for raising the men and the work pieces to where they are needed.
Since the vehicles frequently are operating in areas remote from an adequate electrical power source, the extension and articulation of the equipment is normally carried out by the use of hydraulic pressure systems. The hydraulic system normally comprises at least one hydraulically extensible cylinder in each arm as well as a hydraulic cylinder at the pivot points of the articulated arms in order to provide the necessary articulation. Since the movement of the arms is accomplished by the extension of the piston rod from the hydraulic cylinder, there is no requirement for considerable flexing of connections between the cylinders and the pressure source and hence the hydraulic system is particularly well adapted for such usage. Battery powered systems are known but they are normally actuated with electrical conductors, fiber optics, or hydraulic or pneumatic lines extending between the battery powered system and the work platform, which presents a potential hazard when the work must be accomplished in the vicinity of high voltage transmission lines. Also the flexing of the articulated arm assemblies has imposed problems on the bending of the electrical conduits, fiber optics, or hydraulic or pneumatic lines, resulting in some instances in the use of a reel-like arrangement for the conduits to solve the problem of extension and retraction of the conduits.
The pressure for a hydraulically operated system is provided by a pump which is operated on a power take-off from the vehicle engine. This can be either an arrangement in the engine compartment or in some instances can be a separate transmission shaft which is engaged by controls within the cab of the vehicle and other means of providing the necessary pumping pressure by use of the vehicle's main engine are available. The power take-off pump builds up pressure in the system and in the movement of the articulated arms is accomplished by actuation of control levers which usually are positioned at the end of one of the arms on which there is attached a working platform or basket. These arrangements are standard and are well known in this art. The pump may also be operated by a separate engine in the case of a trailer mounted lift unit.
The disadvantages of the conventional hydraulic system operated by power take-off are to be found in the fact that such is a gas consuming operation since the vehicle engine must continue to operate. An expensive cooling system for the vehicle engine is required to prevent overheating during the long periods of use. The operation of the vehicle engine not only is a pollutant from a noise standpoint but also emits combustion fumes into the air which are particularly noxious since in many instances the vehicle engine is not able to operate at its most economical speed both as far as gasoline supply is concerned as well as a combustion. The conventional system also has a disadvantage in that if the vehicle engine exhausts its fuel or for some reason becomes inoperative, it is not possible to utilize the equipment on the vehicle from the work platform either for the purpose of performing work or for safety purposes in lowering the equipment and men.
U.S. Pat. No. 3,666,046 discloses a portable scaffold which can be placed on the bed of a pick-up truck. The scaffold comprises a mast which can be raised from a horizontal to a vertical position by a first hydraulic means, and the mast then can be raised or lowered vertically by a second hydraulic means. A platform is attached to the mast for workmen. The platform is moved vertically by means of a separate electrically driven pump which is controlled by an electrical conduit extending from the platform down to the pump motor combination.